The present invention relates to methods of determining the standard colour values of colours represented on a colour monitor, of the kind in which the colour monitor is driven by colour measurement value signals, and to apparatus for carrying the method into effect. The determined standard colour values form the starting quantities for calculating the mixing proportions of the colours.
Whereas the multi-colour printing each colour is produced by printing various proportions of the separation colours (magenta, cyan, yellow) one on top of the other, the clours to be printed in decoration, packaging or textile printing are compounded prior to the printing process in accordance with colour originals or specimen designs and are then transferred separately to the printing medium (full area/printing).
Determination of the colour components (the colour formula) constitutes a considerable problem. For each given colour original or model the colour formula should be so calculated that when the colour is imitated the same colour impression is created as in the case of the colour original.
To determine the components of the mixture, use is made, for example, of a colour mixing table or chart, in which the proportions of primary colours which are required for compounding are given for every specimen colour.
The colour original to be printed must be compared with the individual specimen colours of the colour mixing table, an operation which presupposes a very reliable colour gauging ability. If no correspondence of colour can be established when the visual comparison is made, the colour formula must be determined empirically by colour mixing and application, which is very time-consuming.
There are already known, however, so-called colour-formula computers in the form of process computers, which calculate the necessary colour formula as proportions of primary colours from the data fed in.
The specific spectral data of any desired set of primary colours are fed into the colour-formula computer as fixed input values. The colour original for the colour to be compounded must be measured colorimetrically by means of a colour measuring instrument by determining the standard colour values (hues) or the spectral reflectance data. From the standard color values and the specific spectral data of the primary colours, the colour-formula computer then determines the necessary colour formula for a certain type of light.
This method also has the disadvantage that the very accurate color sample must first be available or prepared in order to be able to measure the standard colour values.
Moreover, the possibility of being able to evaluate the colour original to be printed as regards the arrangement of the design and the colour composition of the various coloured areas is lacking.
Moreover, so-called colour-separating systems for colour-true simulation of printing processes are known, by means of which visual evaluation and correction of the final product is rendered possible.
In such a colour-separating apparatus an original is scanned as in television by means of a colour camera and re-recorded on a colour monitor. The colour signals produced during scanning are converted into corrected colour separation signals in a colour computer. A printing simulating computer connected at the output end of the colour computer and which specially takes into account the printing parameters for multi-colour printing transforms the corrected colour separation signals into control signals for the colour monitor in such manner that the representation of the original on the screen of the colour monitor creates the same colour impression as multi-colour printing.
It is true that with these colour-separating systems colour originals and specimen designs for decoration, textile and packaging printing can also be examined for arrangement or form and colour, but even these systems do not solve the problem of indicating the standard colour values for the colours represented on the colour monitor for the purpose of calculating the colour formula. Since the known colour-separating systems are specially designed for the purposes of multi-colour printing, these systems also do not supply any signals from which the standard colour values could be derived.
It is true that colour correction can also be carried out with the known colour-separating systems, but this is operative over the whole of the picture screen; selective colour correction for different coloured areas disposed side by side, such as occur in the colour originals for the types of printing mentioned, is not possible however.
It is therefore an object of the invention to provide a method of and an apparatus for determining the standard colour values of colours represented on a colour monitor without a visual comparison of colours having to take place and wherein the standard colour values form the starting quantities for a colour mixture calculation for the colours represented.